Mention possibility to use Masquerade in a separate project

Masquerade doesn't have to be in the same project as the Jmix application.

This should be mentioned in the documentation. 
To create a separate project with `Masquerade`, simply create **an empty Java project in Gradle** and add the dependency:
```groovy
    testImplementation('io.jmix.masquerade:jmix-masquerade:{VERSION}')
```